California’s new law directing school districts to limit student cell phone use during teaching hours may sound controversial, but it is a big win for child and adolescent public health — and a potentially important step in helping America’s students improve their mental well-being.  The data is clear: Teenagers spend 40 hours on their screens every week. This time spiked during the COVID-19 outbreak when devices became essential learning tools for isolated students.   Since returning to the classroom, however, screen usage has remained constant while K-12 schools continue to see “achievement gaps” in math and reading — disparities that haven’t recovered since the pandemic.  To help students learn, we need to remove the distraction of phones in the classroom.   Nearly 100 percent of middle and high school students admit to using their devices during school hours, and most high school teachers say it’s a “major problem.” Restricting these devices during the school day is one way states can help teachers and administrators keep students focused and engaged socially and academically.  California is the latest among a growing number of states that have taken steps to address this problem. School cell phone policies are gaining traction: As of June, at least 15 states have banned or restricted student cell phone use during school hours. Other states can follow their lead and adopt similar measures to help students focus on their academic work and not their phones throughout the school day.  Teenagers spend upwards of five hours a day on social media, something Surgeon General Vivek Murthy warns can compromise their mental health.  “Adolescents who spend more than three hours a day on social media face double the risk of anxiety and depression symptoms,” he says, adding that “nearly half of adolescents say social media makes them feel worse about their bodies.”   Murthy is pushing for warning labels on social media platforms as part of his campaign to combat loneliness, which he calls a public health “crisis.” Research shows cell phone addiction can actually fuel loneliness; limiting access to social media, even for a little while, can improve adolescent mental health and develop stronger student connections.   There’s evidence these restrictions can lead to positive student outcomes. Norway limited cell phone use in hundreds of middle schools years ago, and it reduced bullying (for both boys and girls) by over 40 percent. Student visits to psychology specialists dropped an astonishing 60 percent while overall grade point averages increased.    Not surprisingly, students are often reflexively opposed to these proposed limitations as they’ve grown accustomed to having immediate access to their devices. But schools that have already implemented phone restrictions in the U.S. have seen encouraging academic and behavioral results in a short period of time.   A high school in the Bronx that instituted a cell phone ban saw Advanced Placement test scores increase — and overall academic performance improve within a year. Attendance at after-hours sporting events rose, too. Some students even thanked the school’s principal for the ban saying it helped them focus more during school hours.   At another high school in upstate New York, classroom engagement improved — and an increase in student socialization took place, evidenced by a louder lunchroom as students could no longer sit and scroll in silence.   The school’s principal said, “In our classrooms, when you would actually speak to them, you had their attention.”  Experts concur that banning cell phones in schools can have a powerful impact on improving student mental health.   “Kids are feeling isolated, lonely, disconnected. They’re also performing poorly in schools,” says Zach Rausch, lead researcher for the book, “The Anxious Generation.” “A large reason for both of these problems [is] the infusion of addictive tech in schools.”  This summer, Sens. Chris Murphy (D-Conn.) and Mitt Romney (R-Utah) introduced the Smart Kids Not Smartphones Act to recognize schools that implement classroom cell phone restrictions.  In announcing the bill, Murphy noted how Connecticut’s efforts to reduce cell phones in schools have positively impacted mental health — adding to the chorus of other states that have seen similar results.   Limiting cell phones at K-12 schools across the country can strengthen teen health, advance student learning and build better school communities. America could use a healthy dose of all of the above.  Lyndon Haviland, DrPH, MPH, is a distinguished scholar at the CUNY School of Public Health and Health Policy.